Dental implants are used to replace individual teeth or for anchoring more complex structures, which generally replace several or even all of the teeth. The materials used for dental implants are often titanium and alloys thereof, and increasingly ceramic materials. These materials have the necessary strength for withstanding the mechanical loads that occur, and they are at the same time sufficiently biocompatible for osseointegration and long term use in the mouth.
Implants are often constructed in two parts, in which case they consist of an anchoring part, often referred to in isolation as the implant, and of a separate abutment. The anchoring part is either embedded completely in the bone, that is to say to the height of the alveolar crest, or protrudes by a few millimeters from the alveolar crest into the soft tissue. The abutment is mounted on the anchoring part either after the latter has become incorporated (osseointegrated) into the bone or directly after the anchoring part has been inserted. It can also be attached to the anchoring part prior to insertion. Most usually the abutment is not mounted until after osseointegration. In such cases a component called a healing cap is often mounted to the implant during the osseointegration process to prevent incursion of soft tissue over the implant site. Ultimately, the desired prosthetic element (e.g. bridge or crown) is connected to the abutment. The prosthetic element can be adhesively bonded, cemented, screwed or directly veneered onto the abutment.
It is also possible for the implant to be constructed in one part, such that the anchoring part and the abutment are produced in one integral piece. Hence in such implant systems the integrated anchoring part and abutment are always positioned within the mouth at the same time.
In contrast to one piece implants, two-part implants are more versatile, because the anchoring part and the abutment can be adapted to individual requirements. In particular the abutment shape and angulation, relative to the anchoring part, can be selected after implant insertion. This provides the surgeon with more flexibility and room for error in the placement of the implant. An additional advantage of two-part implants is that the abutment can be made from a different material than the anchoring part.
Due to their versatility two part dental implants are more commonly used than one-piece implants, and it is this form of implant with which the present invention is concerned. For the remainder of this specification therefore, the term “implant” will be used to denote the anchoring part of a two part implant, namely, the element which in use is anchored within the bone, and the term “secondary component” will be used to denote a component which is, in use, fastened to the implant and which protrudes from this into and in some cases through the gum. The secondary component can be an abutment, which provides support for a dental prosthesis, or in some instances may comprise an integrated abutment and final prosthesis. It can also be a component which is temporarily fixed to the implant such as a healing cap or impression post.
All secondary components must be capable of being firmly fastened to the implant in order to prevent loosening and potential loss of the component. This can be achieved in numerous ways, for example, via compression fit or cementing. However, screw fit connections are generally preferred. By applying a sufficiently high torque during attachment a firm connection between the implant and secondary component can be achieved.
In some systems therefore the implant comprises an internally threaded bore, while the secondary component comprises a corresponding apical thread, thus allowing the secondary component to be screwed directly into the implant. However, this has the disadvantage that the exact angular position of the secondary component relative to the implant is not known until final fixation. This can have disadvantages, particularly in relation to abutments used to support a single tooth prosthesis.
Therefore, many implant systems comprise anti-rotation means, which prevent relative rotation between the implant and abutment, or other secondary component, and which set a finite number of rotational positions which the secondary component can have relative to the implant.
These anti-rotation means consist of complementary non-circular portions in the implant and secondary component, usually having a polygonal shape such as a hexagon or octagon. For example, the internal bore of the implant may comprise a section having a hexagonal cross-section, while the abutment or other secondary component comprises a portion having an equivalent hexagonal cross-section. Alternatively the implant may comprise a male polygonal boss protruding from its coronal end, which in use is accommodated within a correspondingly shaped polygonal cavity within the secondary component.
Such systems ensure that the exact rotational position of the abutment or other secondary component in relation to the implant is known prior to fixation and can help to prevent loosening of the abutment during the lifetime of the implant.
Of course, when such anti-rotation means are employed it is not possible to rotate the secondary component relative to the implant and hence the secondary component can no longer be directly screwed into the implant. Therefore a third component, often a screw known as a “basal screw”, is used to connect the secondary component to the implant.
When a basal screw is used the abutment or other secondary component typically comprises a screw channel extending through the component and having a screw seat. This enables the basal screw to be fed through the secondary component until the screw head abuts the screw seat and for a screwdriver to be inserted into the channel to connect to the screw and fasten this to the threaded bore of the implant.
An example of such a known implant system can be found for instance in US2002/0031748.
One problem with such systems is that the screw channel weakens the abutment by reducing its volume. Although this effect can be reduced by keeping the screw channel width to a minimum, the screw channel diameter is set by the diameter of the screw. This in turn must be sufficiently large to ensure that the screw is strong enough to withstand the torque and bending forces that will be placed on this firstly during assembly and then later during the use of the implant system, when strong masticatory forces will be experienced. This therefore limits the ability to reduce the screw channel width.
The need to prevent interference with existing teeth and to reduce bone loss limits the diameter of the implant (anchoring part) and hence also the apical end of the secondary component which is fastened to the implant. This is especially true when the apical end of the abutment is sized to be inserted into the implant. For this reason, the screw seat is usually positioned higher within the secondary component, where the volume is greater and can thus better withstand the weakening effect of the largest diameter section of the screw channel, namely that above the screw seat. This necessitates a longer screw shaft length and hence higher bending forces being experienced by the screw during use. The width of the screw channel also places restrictions on the minimal shape of the abutment, which limits the possibilities for grinding the abutment into an individualised, patient-specific shape.
The above problems are of particular concern in relation to ceramic abutments. Such abutments have high aesthetic benefits due to their colouring. However, these materials are brittle and prone to chipping, particularly in thin walled areas.
EP2036515 discloses an abutment in which the screw is held within the screw channel of the abutment by a metal inlay, which reduces the diameter of the screw channel above the screw head and prevents the screw from falling out. EP1139906 discloses a similar system in which the inlay is located at the apical end of the screw channel, below the screw head.
In both of these systems the abutment and screw form a three part assembly, wherein the final element, the inlay, is attached only after the screw has been inserted into the abutment. While these systems enable the screw to be retained within the abutment prior to connection to the implant, their ability to reduce the screw channel width and increase abutment strength is limited.
Both of these systems still require the provision of a coronal screw channel having a width great enough to enable a screwdriver head to pass through and engage with a standard basal screw head. Further, the provision of a separate inlay increases the cost and complexity of the abutment. In addition EP2036515 does not solve the problem of decreasing the minimal shape of the abutment, as for stability the inlay must remain encased within the coronal end of the abutment. As regards EP1139906, the inlay effectively forms the screw seat in this arrangement and thus is subject to high tensile stresses which can lead to damage of the inlay.